Verb (5-dan, く, intransitive)
1. to be effective; to take effect; to be good (for)
usu. 効く
あの薬、効かなかったんだろうか？I wonder if that medicine didn't work.
2. to work; to function well
usu. 利く for body senses, functions, etc.
ブレーキが効かなくて、本当に怖かった。The brakes didn't work, and it was really scary.
3. to be possible (to do, use, etc.); to be able to
これは決定事項、変更は効きませんので我慢して下さいね。This has already been decided and can't be changed, so please bear with it.
Verb (5-dan, く, transitive)
4. to taste (alcohol); to try